Superheterodyne automatic gain control system



Dec. 28, 1937. .1. c. NONNEKE NS SUPERHETERODYNE AUTOMATIC GAIN CONTROLSYSTEM Filed June 27, 1954 ATTORNEY rectified by a second detector.

Patented Dec. 28, 1937 UNITED STATES PATENT OFFICE SUPERHETERODYNEAUTOMATIG GAIN CONTROL SYSTEM Application June 27, 1934, Serial No.732,558 In the Netherlands August 3, 1933 4 Claims.

This invention relates to the regulation of the sound strengthinreceiving sets, in which the well known superheterodyne principle lsused.

With this receiving method modulated high 5 frequency oscillations,whether or not amplified by a high frequency amplifier, are caused tointerfere With the electric oscillations produced by a local generator,and subsequently supplied to a first detector by which the beatoscillations are rectified. In this case oscillations of thesum-frequency and also oscillations of the difference-frequency of thereceived and the cally produced oscillations are present in the outputcircuit of the first detector. In superheterodyne receiving sets theoscillations of the difference-frequency are usually employed. Thisso-called medium frequency which has taken over the modulation of thereceived high frequency oscillations, is successively amplified and Thelow frequency oscillations occurring therewith are made perceptible, ifdesired, after having been subjeoted' to low frequency amplification, bysome reproducing device, for instance, a loud speaker. Now, it is knownthat the amplitude of the medium frequency oscillations depends on theproduct of the amplitude of the received high frequency oscillations andthe amplitude of the oscillations produced by the local oscillator. Dueto fading phenomena the amplitude of the received high frequency carrierwave is subject to variations so that the amplitude of the mediumfrequency oscillations, and also the strength of the low frequencysignals made perceptible after the second rectification will varyaccordingly.

The present invention has for its purpose to provide means by whichvariations in the amplitude of the medium frequency oscillations due tovariations of the received carrier wave amplitude are entirely or almostentirely avoided, so that medium frequency oscillations of alwayssubstantially constant amplitude are obtained, due to which the lowfrequency signal strength remains substantially constant independentlyof fading phenomena.

According to the invention this purpose is attained by controlling theamplitude of the oscillations generated by the local oscillator inaccordance with the amplitude of the received oscillations in such amanner that the product of the amplitudes of the local and of thereceived oscillations is substantially constant.

In order that the product of the amplitudes of the local and thereceived oscillations always remains constant, a variation of thecarrier wave amplitude must be incidental to an inversely proportionalvariation of the amplitude of the local oscillations, which consequentlymeans that if, for instance, the amplitude of the received highfrequency oscillations impressed on the first detector, decreases tohalf the initial value, the amplitude of the local oscillations mustincrease two fold.

Such a control of the amplitude of the local oscillations can be ensuredin different manners. It is possible, for instance, to rectify thereceived high frequency oscillations after having been amplified, forwhich purpose either the high frequency amplifier connected before thedetector, or a separate high frequency amplifier, may be used. Thus, adirect current is produced whose value depends on the carrier waveamplitude and by means of the voltage drop caused by this direct currentthrough a resistance, the bias of one or more of the electrodes, forinstance, of 20 one of the grids or of the anode of the tube of thelocal generator can be controlled.

Another possibility of ensuring the desired control is to regulate bymeans of the voltage drop caused through a resistance by the direct cur-25 rent set up by rectification of the medium frequency oscillations inthe second detector, the bias of one or more of the electrodes of thetube of the local generator. In this case it is possible, of course, touse a separate rectifier instead of the second detector.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims. The inventionitself, however, both as to its organization and. method of operation,will best be understood by reference to the following description takenin connection with the drawing, in which I have indicateddiagrammatically a circuit organization whereby my invention may becarried into effect.

One embodiment of the invention will be more clearly set out byreference to the accompanying drawing in which a superheterodyne set isdiagrammatically represented.

The input terminals of a high frequency amplifier H. V., the circuitarrangement of which may be supposed to be known, are connected to anantenna A and to earth E. The amplified received high frequencyoscillations are impressed on the input circuit of a first detector D1to which are also supplied in a suitable manner the oscillationsgenerated by a known local oscillator L. 0., so that a medium frequencyis produced which after having been amplified in a medium frequencyamplifier M. V. is rectified by a second detector D2. successively thelow frequency signals can be amplified by a low frequency amplifier L.V. and rendered perceptible by a suitable reproducing device I.

As appears from the drawing the second detector D2 may be constituted bya thermionic tube I, of the multi-duty type, comprising an anode 2, acontrolling grid 3, an indirectly heated cathode 4 and in addition asmall anode 5 outside the electron stream to the grid 3 and anode 2. Theamplified medium frequency oscillations are sup plied between the anode5 and the cathode 4, for instance, by means of a tuned circuit 6. Sincethese electrodes form a diode-rectifier, the resistance 'I will betraversed by a rectified current comprising both low frequencycomponents and a direct current component. In order to allow the passageof the high frequency alternating currents a condenser 8 is connected inparallel with the resistance 7. The low frequency alternating tensionsthrough the resistance I control the con trolling grid 3 of the tube Ibefore which a condenser 9 together with a leakage resistance I isprovided. Now the low frequency anode current variations brought aboutby this control means cause voltage variations across a resistance I Iinserted in the anode circuit of the tube I, by which variations the lowfrequency amplifier is controlled.

The arrangement of the local oscillator represented in the drawingcomprises a triode I2 which is coupled back and which has a tuned gridcircuit I3 to which a coil I4 is coupled whereby the generatedoscillations are impressed on the first detector D1. In order to achievethe desired variation of the amplitude of the local oscillations aresistance I6 is inserted in the anode circuit of the tube I2 in serieswith the reaction coil I which resistance is shunted by a condenser l1having a small impedance -to high frequency currents. This resistance IBis at the same time in series with a source of anode voltage I8 in theanode circuit of a tube I 9 whose grid 2!! is controlled by the directvoltage drop through the resistance 1 with the interposition of a filterF suppressing the low frequency currents.

The working of the arrangement is as follows: With a definite strengthof the incoming signal a direct voltage drop having a correspondingvalue occurs through resistance I. Accordingly, an anode current flowsin the tube I 9, which current causes a certain voltage drop through theresistance I6. This voltage drop together with the voltage of the sourceof voltage I8 determines the anode voltage of the oscillator tube I2 andthis anode voltage is adjusted in such a manner that with a normalsignal strength the alternating tensions supplied by the localoscillator to the first detector have a definite desired value. If, forinstance, the amplitude of the incoming carrier wave decreases then thevoltage drop through the resistance 1 willalso decrease. Due to this thegrid 2!] of the tube I9 acquires a more negative voltage relatively tothe cathode, so that the current through the resistance I6 willdecrease; thus the anode voltage of the oscillator tube I2 willincrease. Assuming the amplitude of the oscillations generated by theoscillator to be in a linear relation with r the anode direct voltage,which may be ensured by giving the oscillator a proper size, then anincrease of the anode voltage of the tube I 2 will cause an increase ofthe amplitude of the local oscillations, and consequently also anincrease of the voltages supplied to the first detector.

If the increase of the last mentioned voltages is almost inverselyproportional to the decrease of the amplitude of the receivedoscillations at the first detector due to the decrease of the signalstrength, then the amplitude of the medium frequency oscillations willremain substantially constant, since as is well known the mediumfrequency amplitude depends on the product of the amplitudes of localand received oscillations. In a quite analogous manner can be shown thatif the strength of the incoming signals increases, the amplitude of thelocal oscillations will decrease so that also in this case the amplitudeof the medium frequency oscillations remains substantially constant.Thus, with a constant modulation degree of the received transmitter thelow frequency signals supplied to the low frequency amplifier areautomatically kept on a constant level so that annoying phenomena due tofading are no longer perceptible,

The sound volume control according to the invention may be usedsimultaneously with known sound strength regulations. This, maybeadvantageous, for instance, if a control between very wide limits isdesired. With the circuit arrangement represented in the drawing thismay be ensured without any further means by causing the direct voltagedrop to act through the resistance 1 on the controlling grids of the,high and/or medium frequency amplifying tubes.

If it is desired that the automatic sound strength control becomesoperative only when the received signal strength exceeds a definitevalue, this may be ensured in a simple manner with the arrangementrepresented in the drawing by giving the grid 20 of the tube I9 anegative bias from a source as 30 having such a value that anode currentcan flow in the tube I9 only if the signal strength exceeds a definitevalue.

Regulation of the amplitude of the local oscillations occurs in thearrangement shown in the drawing by varying the anode voltage of theoscillator tube. It will be appreciated, however, that this purpose canalso be attained in various other manners, for instance, by varying thebias of the controlling grid, or when using a tube having a plurality ofgrids, by varying the bias of one of the remaining grids.

While I have indicated and described a system for carrying my inventioninto effect, it will be apparent to one skilled in the art that myinvention is by no means limited to the particular organization shownand described, but that many modifications may be made without departingfrom the scope of my invention, as' set forth in the appended claims.

What is claimed is:

1. In a signal receiver, a frequency changer network adapted to havesignals impressed thereon, a local oscillator coupled to the saidnetwork and impressing current thereon of a frequency differing from thesignals by a desired beat frequency, a detector adapted to have the beatcurrent impressed thereon, and means, responsive to the variations inthe direct current component of the detected beat current, for varyingthe amplitude of the oscillator current in a direction such that theamplitude of the beat current is substantially constant, said last meansincluding an electron discharge tube having its input electrodesarranged to have the said component impressed therebetween, and animpedance common :7 5

to the space current paths of said tube and the local oscillator.

2. In a superheterodyne receiver of the type including in cascade asignal amplifier, a first detector, an intermediate frequency amplifier,a second detector and an audio frequency amplifier, a local oscillatorcoupled to the first detector, means for automatically regulating theamplitude of the local oscillations impressed on the first detector in asense inverse to amplitude variations of signal waves received by thereceiver, said means comprising an electron discharge tube having animpedance in its space current path, said impedance being included inthe oscillator space current path, and connections between the inputelectrodes of said tube and the second detector for impressing on theinput electrodes the direct current component of detected intermediatefrequency current.

3. In a superheterodyne receiver of the type including in cascade asignal amplifier, a first detector, an intermediate frequency amplifier,a second detector and an audio frequency amplifier, a local oscillatorcoupled to the first detector, means for automatically regulating theamplitude of the local oscillations impressed on the first detector in asense inverse to amplitude variations of signal waves received by thereceiver, said means comprising an electron discharge tube having animpedance in its space current path, said impedance being included inthe oscillator space current path, and connections between the inputelectrodes of said tube and the second detector for impressing on theinput electrodes the direct current component of detected intermediatefrequency current, and additional connections from the second detectorto the signal and intermediate frequency amplifiers for impressingthereon the said component thereby to vary the gain of the said lastamplifiers.

l. In a signal receiver of the superheterodyne type, a first detectornetwork, a local oscillator network electrically associated with thedetector to produce energy of a predetermined intermediate frequency inthe detector output circuit, said oscillator network including anelectron discharge device having at least a cathode, control grid andanode, means, responsive to variations in received signal amplitude, foradjusting the oscillator anode potential in a sense to maintain the saidintermediate energy at, a substantially constant amplitude, andadditional means for delaying the adjusting action of said means untilthe received signal amplitude exceeds a desired value.

JACOBUS C. NONNEKENS.

